The present invention is directed to subtraction radiography, and more particularly, is directed to apparatus for taking radiographs used in performing dental subtraction radiography in real time, with a sensorized dental mouthpiece and a robotic system.
Radiographs are one of the most widely used diagnostic tools in dentistry. This remains true in spite of the inherent limitation that they provide only a two-dimensional projection or view of the area of interest.
However, as dental health has improved, the emphasis has shifted toward early detection of disease, requiring more exacting instruments. Nowhere is this more evident than in the diagnosis and treatment of periodontal disease. The goal is to be able to detect, as early as possible, small changes in the bony structure supporting the tooth. Studies have shown that disease can be detected earlier and with greater accuracy by looking at the difference between radiographs (x-ray films) taken over time, instead of a single radiograph. This "subtraction" technique can be accomplished optically by aligning films and viewing the same, or digitally, by digitizing each film and then subtracting corresponding picture elements to compute a difference image.
Digital subtraction radiography is one example of an imaging application which requires a measurement of change over time. Two images, each of which records the state of the dentition (teeth), are taken at different times. Since the normal anatomical structure should not change between the two films, the observed differences are indicative of growth or decay in the bony structure. The early studies used subtraction to detect lesions in between teeth and in the supporting structure of the teeth. See L. Ortman et al., "Subtraction radiography and Computer Assisted Densitometric Analyses of Standardized Radiographs" Journal of Periodontal Research. Vol. 20, pages 644-651, 1985; and M. Rethman et al., "Diagnosis of Bone Lesions by Subtraction Radiography" , Journal of Periodontal Research, Vol. 56, pages 324-329, 1985. More recent papers cite applications of digital subtraction to measure bone loss (change in density) in the supporting structure of the teeth. See U. Braegger, "Digital Imaging in Periodontal Radiography", Journal of Clinical Periodontoloov, Vol. 15, pages 551--557, 1988; U. Braegger et al., "Color Conversion of Alveolar Bone Density Changes in Digital Subtraction Images", Journal of Clinical Periodontoloogy, Vol. 16, pages 209-214, 1989; U. Braegger et.al., "Remodeling of interdental alveolar bone after periodontal flap procedures assessed by means of computer assisted densitometric image analysis (CADIA)", Journal of Clinical Periodontology, Vol. 15, pages 558-564, 1988; and U. Braegger et.al., "Computer-Assisted Densitometric Image Analysis for the Assessment of Alveolar Bone Density Changes in Furcations", Journal of Clinical Periodontoloov, Vol. 16, pages 42-52, 1989.
However, these techniques present a unique imaging problem. The two radiographs must enclose the same field of view of the mouth and must be taken with the same geometry. In other words, the radiographs taken at different times must be in exact alignment, or stated otherwise, the x-ray films must be positioned at the identical location in the mouth and the x-ray gun and film must be aligned at the identical position and orientation for the different radiographs taken at different times.
The purpose of this subtraction is to eliminate the anatomic features not of interest, that is, the so-called "structured noise" in the image. However, if the radiographs are not in alignment, then the radiographs cannot be subtracted meaningfully. Specifically, if the imaging geometry is not the same, then the visual appearance of the structured noise is different and the difference will not be zero, yielding a false positive difference.
Thus, in all cases discussed above, the articles point out the difficulty in making the initial measurements, the difficulty in reproducing the original imaging geometry, and why this limits the use of digital subtraction radiography. The goal must therefore be to produce standardized views of the area of interest.
Much work to date has been done, using mechanical devices to fix the patient and the x-ray source at fixed positions. The proposed approach is based on accurately positioning the x-ray source, and then recognizing three-dimensional properties of the patient that are invariant in any view of the patient.
Thus, for example, the requirements of fixing the geometry to produce standardized radiographs have been recognized, by using a template to fix the mouth in position. See R. Webber et.al., "X-ray Image Subtraction as a Basis for Assessment of Periodontal Changes", Journal of Periodontal Research, Vol. 17, pages 509-511, 1982. Mechanical devices to standardize the imaging geometry have been used elsewhere. See, for example, H. Grondahl et.al., "Subtraction radiography for diagnosis of periodontal bone lesions", Oral Suroerv, Vol. 55, No. 2, pages 208-213, 1983; H. Grondahl et.al., "A digital subtraction technique for dental radiography", Oral Surqerv, Vol. 55, No. 1, pages 96-102, 1983; P. Janssen et.al., "The Detection of In Vitro Produced Periodontal Bone Lesions by Conventional Radiography and Photographic Subtraction Radiography Using Observers and Quantitative Digital Subtraction Radiography", Journal of Clinical Periodontoloqv, Vol. 16, pages 335-341, 1989; P. Janssen et.al., "The Effect of In-Vivo-Occurring Errors in the Reproducibility of Radiographs on the Use of the Subtraction Technique", Journal of Clinical Periodontoloov, Vol. 16, pages 53-58, 1989; and K. McHenry et.al., "Methodological Aspects and Quantitative Adjuncts to Computerized Subtraction Radiography", Journal of Periodontal Research, Vol. 22, pages 125-132, 1987. The articles all used subtraction in trials to quantitate bone loss. In the aforementioned article "The Effect of In-Vivo-Occurring Errors in the Reproducibility of Radiographs on the Use of the Subtraction Technique", Janssen et.al. found that the digital subtraction system was the most sensitive to measure subtle changes when compared to using a single radiograph or photographic subtraction, but required standardized geometry.
In the aforementioned article by Webber et.al., the authors noted that there were four sources of error leading to improper registration of a pair of radiographs, namely tissue changes, film, x-ray energy and inexact replication of imaging geometry. The first cannot be controlled other than by assumptions on the localization of the changes. The second can be controlled by the film type and processing chemicals. As to the third, Webber later studied the effects of polychromatic x-ray energy and showed that the effects of energy can be controlled. See R. Webber et.al., "The Effects of Beam Hardening on Digital Subtraction Radiography", Journal of Periodontal Research, Vol. 24, pages 53-58, 1989. The remaining problem therefore is that of controlling the imaging geometry.
To date, most approaches to this problem have relied on a model of radiograph formation of a point projection of x-rays along straight lines through the tissue. The x-rays are attenuated along these diverging straight line paths and form a distorted image on the film behind the hard tissue. As the source and/or film move with respect to the tissue, the appearance of the tissue on the film is changed non-linearly. Thus, to generate radiographs with the same appearance, it is important to reproduce the original imaging geometry. Following this line of reasoning, many studies of the subtraction technique have been reported that use mechanical fixtures to control the imaging geometry, that is, the position of the point source and the attenuation paths through the tissue. See, for example, K. Grondahl, "Influence of Variations in Projection Geometry on the Detectability of Periodontal Bone Lesions", Journal of Clinical Periodontology, Vol. 11, pages 411-420, 1984; the aforementioned second Janssen article at pages 53-58; M. Jeffcoat et.al., "A New Method for the Comparison of Bone Loss Measurements on Non-Standardized Radiographs", Journal of Periodontal Research, Vol. 19, pages 434-440, 1984; the aforementioned K. McHenry et.al. article; and the aforementioned Webber et.al. article at pages 509-511. Other studies have used compensatory algorithms to control the imaging geometry. See, for example, the aforementioned Jeffcoat et.al. article and P. van der Stelt, "Determination of Projections for Subtraction Radiography Based on Image Similarity Measurements", Dento Maxillo Facial Radioloov, Vol. 18, pages 113-117, 1989.
Thus, for example, a good example of a mechanical standardization system for subtraction radiography is given in the aforementioned Jeffcoat et.al. article. As explained therein, the x-ray source, patient and film are connected using an occlusal stent (bite block between the teeth) and cephalostat (fixture for the patient's head). The mechanical connection of the source, patient and film should restrict any variations to be in plane translations and rotations. By matching three anatomical features in the two films, the translations and rotations can be eliminated. The difficulty with these systems is two-fold. First, the use of a mechanical fixture or fixed imaging geometry provides that the field of view is restricted. In subtraction radiography, this means that only a limited portion of the dentition can be imaged. Secondly, disease processes are three dimensional, and not two-dimensional.
Various patents are known which disclose systems that are relevant to the present invention.
U.S. Pat. No. 4,223,228 to Kaplan discloses a dental x-ray aligning system having a permanent magnet mounted on the dental appliance or mouthpiece so that the magnet is in a fixed relation with respect to the x-ray film plate. The system also includes a sensor, in the form of Hall effect devices or probes, on the x- ray source in order to align the film and the source, as well as an indicator circuit. Basically, the indicator circuit, in response to the Hall effect probes, indicates the direction in which the apparatus should be moved in order to obtain alignment, as well as the distance between the apparatus and film plate.
However, the orientations permitted by this system are only translational orientations, and not rotational orientations. As a result, a truly accurate control cannot be achieved. Specifically, by using only a permanent magnet, only static alignment can be achieved source is performed manually, requiring human reading of a display screen. There is no inference of automatic orientations of the x-ray source and film without human intervention.
Still further, this system could not be utilized to detect the identical x-ray position at a subsequent time. This is because the receptacle for the x-ray film is clenched between the teeth of the user. Therefore, subsequent measurements may result in the patient holding the receptacle at a slightly different location, which would give a different reading. In other words, the permanent magnet is not fixed at a set position relative to the dentition, and therefore does not preserve the desired relationship with the teeth.
U.S. Pat. No. 4,197,855 to Lewin discloses a system for measuring the location, attitude and/or change in location of a body in space, and particularly, the position of the lower jaw or mandible with respect to the head. With this system, a permanent magnet which serves as a field generator is attached intraorally at any desired point of the lower jaw of the patient by means of an adhesive. An arrangement of several mutually perpendicular magnetic field sensors is mounted on the patient's cranium via a mounting so as to remain locationally fixed vis-a-vis the lower jaw and so as to provide a reference position as the latter is moved relative to the cranium.
Although this system can measure the position and orientation of the mandible with respect to sensors on the head, the system, as with Kaplan, uses a permanent magnet so as to suffer from the same problems. Also, this system is only applicable to the mandible, and not the maxillary arch. Specifically, this system is intended to be used for determining changes in cranio-facial attitude such as the amount by which the mandible opens with respect to the maxillary arch. There is no disclosure of extending this system for use with a radiologic system. Furthermore, this system is a measurement system, and does not provide any control of movement of any device, such an x-ray source, in response to the measurements.
U.S. Pat. No. 3,822,694 to Mills describes a similar system for measurement of jaw opening and orientation. Again, the measurements are made with respect to the cranium by the use of a mechanical attachment to a pair of sunglasses. The system is therefore provided only for measurement, and not for controlling any device, such as movement of an x-ray source. Furthermore, a permanent magnet is attached to the patient's gum using wax so that repeated measurements cannot be made. In sum, this system is only intended to measure gross cranio-facial motion parameters, and not to preserve sensor-tooth geometry.
U.S. Pat. No. 4,295,050 to Linden discloses a mechanical radiograph alignment system for positioning an x-ray camera in dental x-ray photography. Specifically, a bite block is connected to an arm perpendicular to the block, and a second guide is attached to the x-ray source that connects to the bite block to preserve the source-film orientation. However, this system uses a mechanical alignment which does not permit real time control, nor conservation of source-tooth geometry. Although the bite block and arm have marking symbols to make it possible to x-ray exactly the same area, the dental practitioner must make a note of the location of the bite block in relation to a suitable tooth by establishing which symbols are close to that tooth, which is not sufficiently exact during successive uses thereof. In other words, the system is entirely manual, and not automatic.
U.S. Pat. No. 1,719,106 to Cressler only discloses a system that assures repositioning of the x-ray film to tooth, by use of a mouth appliance. Specifically, a mass of material which becomes plastic when heated above normal body temperature but which is non-plastic and firm at or below the temperature of the body, is secured to the free end of the supporting wing of the main body that holds the x-ray film. In use, the patient bites down upon the mass and then the appliance is removed. The mass then cools so as to provide for accurate repositioning of the appliance in the patient's mouth at all times. The patent describes the device as having particular utility in stereoscopic work when it is essential that a plurality of views be taken of an object at different angles, or to take views illustrating the conditions in a particular tooth or a section of the jaw of a patient at different times, for comparison to determine the results of a course of treatment being pursued. However, this system is purely passive and does not have any means for controlling movement of an x-ray source.
U.S. Pat. No. 4,907,251 to Mork et.al. discloses a patient positioning device in a medical panorama x-ray photographing apparatus. The system includes a position detecting sensor which detects the relative position of the dental arch of a patient to the x-ray photographing apparatus and a drive circuit for moving a tomograph forming assembly and/or a patient holding means in accordance with comparative data between the detected position data from the sensor relative to the position data of the tomographic zone to the x-ray photographing apparatus. However, there is no suggestion as to how to reposition an x-ray source with respect to dentition of interest. Furthermore, the installation is mechanical, and requires restraining the head of the patient.
U.S. Pat. No. 2,846,587 to Thurow discloses a cephalostat, commonly used in orthodontics. The patient is positioned and repositioned mechanically, by fixing the position of the bridge of the nose and the ears. Also, a standard x-ray source is used, and not one with automatic robotic control.
Other prior art of interest is found in U.S. Pat. Nos. 4,012,638 (Altschuler et.al.), 4,262,306 (Renner) and 4,887,286 (Seidenberg).
However, none of the above-discussed patents uses a magnetic sensor that permits static reposition in three dimensions. Furthermore, all of these installations are manual, and none provide for automatic control. Still further, none of the patents fixes the sensor-tooth-film geometry to a mouthpiece, nor fixes the x-ray source-tooth-film geometry. More importantly, none of the above-discussed patents can provide suitable resolution of the measurements needed to be made in today's dental environment.